The objective of this research proposal is to define the importance of activation of the alternative pathway (A.P.) of complement in human diseases, especially in systemic rheumatic diseases and allergy. We have devoted our efforts to the physiochemical characterization of certain activators of the A.P. in the sera of patients with systemic lupus erythematosus and we presented some studies which demonstrate a good correlation between the presence of these serum factors and in vivo activation of factor B, a protein of the A.P. of complement. Now we are studying the mechanism of activation of complement by DNA and anti-DNA. We are planning to carry out studies on patients with chronic idiopathic urticaria, trying to define the incidence of complement activation and later on some metabolic studies. We also intend to increase the population of C4 deficient rats which will be used to define the role of the early complement components during activation in vivo and in vitro of the complement system.